What steps should be taken to troubleshoot a 98 Jimmy not blowing air to the floor vents?

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probably hvac mode door stuck (foam or other) or mode door actuator broken - (cheap plastic) - this is an ugly job - find someone who has removed the dash in this type of vehicle before - set aside 300 bucks or more :(

Answer

probaly just needs a blend door acuator.gmjim

ANSWER

Air door controlled by the red and blue vac lines is broken (common fault). Default position is with all air going to defrost vents. See Edmunds.com forums for GMC climate control problems for more help.

Answer

I own a 98 Chevy S10 that is exactly like the Jimmy (just different body styles S10 being a truck and Jimmy an SUV). I had a similar problem to yours, I was barely getting any air from my vents. I fixed it by flushing my coolant system with a flush system I bought at wal-mart that used an ordinary water hose to flush the system. After the flush my heater not only blew air better, but it got hot faster too.

I have owned two mid 90's GM S-10's with this problem. If you can hear the sound of air whistling behind the vent mode selector knob (sound varies as u torn the knob) your problem is there. The selector knob actually routes vacuum thrue the back side by opening and closing small passages on the back side of the knob. The rubber gets old and dirty and begins to leak. Replacing this switch was the cure for me. Or if you like to tinker and u r sharp, take it apart and clean it.